In recent years, there has been known a low pressure EGR device (LPL-EGR system) which serves to introduce a part of an exhaust gas flowing downstream of a turbine of a centrifugal supercharger into intake air upstream of a compressor thereof. In an internal combustion engine provided with an LPL-EGR system, when condensed water existing in a path of an EGR gas comes in contact with the EGR gas, the hydrogen ion exponent or power of hydrogen ions (pH) of the condensed water may be dropped (or strongly acidized or oxidized).
As a result, there is a possibility that the above-mentioned condensed water and/or substances evaporated from the condensed water may cause chemical changes on component parts such as a compressor, an intercooler, and so on. On the other hand, there has also been proposed a technique in which an addition valve for adding a neutralizing agent is arranged in an intake passage at a location upstream of a compressor (for example, see a first patent document).